Nowadays power transmission systems based on inductive coupling are becoming increasingly popular to charge a wide variety of battery powered devices, ranging from handheld electronics, such as cell phones, tablets and computer mouses, to electric vehicles. Key characteristics for such systems are ease of use, high energy transfer efficiency, short charging time and low-cost.
On the other hand, there are several applications in which an electronic device cannot use batteries as a primary source of energy. Implantable devices and smart animal research systems are examples of such applications. In these systems, power is delivered wirelessly in air or across the skin through an inductive link formed by mutually coupled coils to limit risks of infection and any dangerous tethering associated with transcutaneous wires.
Increasing power transfer efficiency (PTE) and improving robustness of such links contribute to the development of several useful applications, such as various types of battery-less microsensors.
Multicoil topologies, for example, three-coil and four-coil topologies, have recently demonstrated higher PTE over longer separation distances. Moreover, multicoil structures are known to provide more degrees of freedom, and can compensate for effects of low coil coupling coefficient (k), and low coil quality factor (Q), which greatly facilitates optimization of the power link. Additionally, multicoil links provide better immunity to variation of the operating frequency.
However, it is established that achieving excellent PTE and high power delivered to the load (PDL) commands the size of the Transmit (TX) coil to be determined based on the size of the Receive (RX) coil and a set of rules. Therefore, arrays including several unit size TX coils have been utilized to transmit power and provide free positioning to a smaller RX coil, without compromising PTE and PDL. Such power transmission arrays have used different types of coil arrangements, including structures made of an array of several individual 2-coil overlapping inductive links to provide a uniform electromagnetic field above a surface, and resonance-based arrays made of several non-overlapping floating coils. In the latter array structure, magnetic coupling propagates all along the array through adjacent coils.
Different techniques have been used to avoid driving every coil of an array at the same time to save power as well as to increase PTE. A magnetic sensor can be used to detect the location of a small magnet enclosed with the receiver. Then, a dedicated control system activates the subset of coils that encompasses the detected magnet to power up the device attached to the receiver. Frequency selection can be employed to localize transmitted power through a subset of active coils towards the receiver. Thus, each coil of the array is tuned to a different resonance frequency, which is challenging to implement and yields limited efficiency, since the resonance frequency on the RX side is fixed and cannot track the selected frequency on the TX side.